Access by user devices to a shared radio access channel by which user devices seek to establish a communications link is a well-known problem. In conventional cellular systems such a shared radio access channel is often the random access channel (RACH), but shared radio access channels can also be used for other purposes such as for example sending device-to-device (D2D) beacons, data/user traffic, or connection requests, or requesting a transmission resource for sending data in a wireless local area network (WLAN) that utilizes license-exempt spectrum. In a distributed way, the mobile radio device (more generally, a user equipment UE) can autonomously sense the channel before transmission to check that no other UEs are using it. However, this sensing does not completely resolve the potential collisions when other UEs are doing the same, although it does reduce the probability of collision. Another technique for UEs to access a shared radio access channel is to have the radio network supervising the process in a centralized way such that the UE uses the channel resources only upon authorization from the network.
Many WiFi-type radio technologies utilize distributed carrier sense multiple access, either with or without collision detection and avoidance. In LTE, a contention-based and a non-contention-based solution are available. For both of these solutions in LTE a total of 64 preambles per cell are available to minimize the probability of collision. With contention based channel access the UE randomly picks one of these preambles. In case of collision between UEs transmitting their randomly selected preambles at the same instant there are other mechanisms such as backoff periods to help resolve this; this is an example of distributed control. With non-contention based channel access the network indicates to the UE which preamble the UE should use for its uplink transmission. Having the network coordinate and assign preambles is an example of centralized control.
The distributed solution has obvious advantages in terms of simplicity, but it is less effective in avoiding collisions which can be a critical factor where a very large number of UEs try to access the radio channel at the same time. The centralized solution more effectively avoids collisions, but with an accompanying cost in complexity and average delay due to required signalling to/from the UEs. While the outage delay may be under control in the centralized case, in the distributed case there is the potential for unbounded delays in the UE gaining access to a channel.
Presently many wireless radio devices communicate with each other while moving from one place to another. The communications between the devices that involve human interaction are known generally as D2X communication which includes device-to-infrastructure (D2I) as well as device-to-device (D2D) communication. The communication between any devices which do not necessarily need human interaction is generally known as machine-type communication (MTC) or machine to machine (M2M) communication. MTC to/from a communication devices embedded in the vehicles are specifically known as vehicular communications V2X and includes both vehicular-to-infrastructure (V2I) and vehicle-to-vehicle (V2V), which along with MTC in general is expected to become more common with the development of 5G radio access technologies. Whether MTC or D2X, sometimes there is a large number of devices in a relatively small region that are trying to communicate at about the same time; for example commuters and/or their vehicles during rush hour. When many such densely packed communication devices are seeking channel access the problem of collisions on the random access channel (RACH) or on other radio channels becomes most acute. Embodiments of these teachings that are more particularly detailed below address these and other problems.
The following references provide a more thorough background that may be relevant to these teachings:    H. Rodziewicz, Location-based mode selection and resource allocation in cellular networks with D2D underlay, 21st European Wireless Conference, May 2015.    H. Kalbkhani et al., Resource allocation in integrated femto-macrocell networks based on location awareness, IET COMMUNICATIONS MAGAZINE.     H. Wymeersch, Location-awarenesss in 5G Wireless Networks, THE FIFTH NORDIC WORKSHOP ON SYSTEM AND NETWORK OPTIMIZATION FOR WIRELESS.     Di Taranto, Location-aware Communications for 5G Networks, IEEE SIGNAL PROCESSING MAGazine 2014.    ETSI EN 302 637-2, Intelligent transport system (ITS); vehicular communications; basic set of applications; part 2: specification of cooperative awareness basic service, 2009.    SAE J2735 specifications [aka, J2735], SAE International, DSRC Implementation Guide, November 2009.    ICT-317669-METIS/D1.1, METIS deliverable D1.1 Scenario, requirements and KPIs for 5G mobile and wireless system, 2013.    M. G. Di Benedetto, T. Kaiser, A. F. Molisch, I. Oppermann, C. Politano, D. Porcino (editors), UWB communication systems—A comprehensive overview, HINDAWI PUBLISHING CORPORATION, 2006. (see Section 4.3 “Location-aware UWB networks”).